Fick's Law of Diffusion (Edexcel AS Biology (A) SNAB): Revision Note
Exam code: 8BN0
Fick's Law of Diffusion
Fick's Law relates the rate of diffusion to the concentration gradient, the diffusion distance and the surface area
This relationship can be represented by the following equation, where ∝ means "proportional to"
rate of diffusion (surface area x concentration difference)thickness of membrane
Proportionality means the rate of diffusion will double if:
The surface area or concentration difference doubles
The diffusion pathway halves
Fick's Law can be written as an equation which can be used to calculate the rate of diffusion
Rate = P x A x ((C1 - C2)T)
Where
P = A permeability constant that is a quantitative measure of the rate at which a particular molecule can cross a particular membrane
A = surface area
C1 - C2 = the difference in concentration between two areas
T = thickness of the exchange surface
Worked Example
A sample of alveolar epithelium tissue from a mammal is 1.5 m thick and has a surface area of 3 m2. The concentration of oxygen in the alveolus is 1.8 x 10-16 mol m-3 and the concentration of oxygen in the blood is 7.5 x 10-17 mol m-3. The permeability constant for oxygen across alveolar epithelium is 0.012 molecule s-1.
Calculate the rate of diffusion across the section of alveolar epithelium.
Answer:
Step 1: Substitute numbers into the equation
Rate = P x A x ((C1 - C2) T)
Rate = 0.012 x 3 x ((1.8 x 10-16 - 7.5 x 10-17)1.5)
Step 2: Complete the calculation
Rate = 0.012 x 3 x (1.05 x 10-16 1.5)
Rate = 0.012 x 3 x 7 x 10-17
Rate = 2.52 x 10-18 molecules m-2 s-1
Examiner Tips and Tricks
You will be given the equation for Fick's Law in your exam, but it is important you understand what it means and how to interpret it.
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